1. Field of the Invention
The invention relates generally to thermal processing of semiconductor substrates. In particular, the invention relates to laser thermal processing of semiconductor substrates in which a line beam is scanned across the substrate.
2. Description of the Related Art
The fabrication of semiconductor integrated circuits has long required multiple steps of thermal processing a silicon wafer or other semiconductor wafer. The wafer may need to be raised to a temperature of 600° C. or well above to thermally activate a process. Such processes, though not so limited, may include chemical vapor deposition, silicidation, oxidation or nitridation, implant anneal, and dopant activation among others. Some of these processes may require temperatures in excess of 1000° C., 1200° C., or even above 1350° C., the last of which is relatively close to the melting point 1416° C. of silicon.
Such thermal processing was originally performed in ovens typically containing many wafers supported in a fixture. Electrical power is applied to resistive heater elements in the oven walls to heat them to a temperature close to the desired processing temperature. The wafers eventually assume a temperature substantially equal to that of the oven walls. After the desired length of thermal processing at the elevated temperature, no more power is applied to the resistance heaters so that the walls gradually cool, as do the wafers. Both the heat-up rates and the cool-down rates are relatively slow, on the order of 15° C./min even though the required thermal processing time may be relatively short. Such long periods at elevated temperatures substantially increase the thermal budget required for thermal processing. The fine features and thin layers in advanced integrated circuits require that the thermal budget be reduced.
Rapid thermal processing (RTP) has been developed to increase the cooling and heating rates. An RTP chamber typically contains a large number of high-intensity halogen lamps directed at a single wafer. The lamps can be quickly turned on to their highest filament temperature to quickly heat the wafer with little heating of the chamber itself. When the lamps are turned off, the portion of the lamps at the highest temperature constitutes a relatively small mass, which can quickly cool. The RTP chamber walls are not heated to very high temperatures. As a result, the wafer can radiatively cool at a reasonably high cooling rate. A typical RTP heat-up rate is about 250° C./s and a typical RTP cool-down rate is about 90° C./s, thus drastically reducing the thermal budget. In a technique called spike annealing, there is essentially no soak time at the maximum temperature. Instead, the ramp up is immediately followed by a ramp down. In most situations, the ramp rates should be maximized.
However, the cooling and heating rates of RTP and even spike annealing are becoming insufficient for advanced devices having ultra-narrow features and shallow and abrupt junctions, both of which require precise thermal control. Both ovens and RTP heat an entire wafer to the required processing temperature. In fact, only the upper few microns of material at the wafer surface require thermal processing. Furthermore, the blanket thermal irradiation pattern of RTP requires cooling of the entire wafer from the annealing temperature, both by radiative and conductive heat transfer. The radiative cooling becomes less effective as the wafer cools.
Pulsed laser thermal processing has been developed to dramatically increase the heating and cooling rates. Short (about 20 ns) pulses of laser radiation are focused at a reduced area of the wafer, ideally the same size as the optical stepper field in the neighborhood of 20 mm by 30 mm. The total energy of the laser pulse is sufficient to immediately heat the surface of the irradiated area to a high temperature. Thereafter, the small volume of heat generated by the shallow laser pulse quickly diffuses into the unheated lower portions of the wafer, thereby greatly increasing the cooling rate of the irradiated surface region. Several types of high-power lasers can be pulsed at a repetition rate of hundreds of pulses per second. The laser is moved in a step-and-repeat pattern over the surface of the wafer and is pulsed in neighboring areas to similarly thermally process the entire wafer surface.
Pulsed laser thermal processing, however, presents uniformity problems arising in part from the short, intense radiation pulses on a patterned surface. The scanning and pulses need to be carefully aligned and neither the radiation profile nor the lateral heat diffusion pattern is flat. The radiation pulse is so short that any difference in absorption will result in a large difference in temperature. One portion of the structure may melt while another portion a micron away is barely heated. To address this problem, a new class of laser thermal processing equipment has been developed in which a narrow line beam of continuous wave (CW) laser radiation having a long dimension and a short dimension is scanned over the wafer in a direction along the short dimension, that is, perpendicular to the line. The line width is small enough and the scan speed high enough that the scanned line of radiation produces a very short thermal pulse at the surface, which thereafter quickly diffuses vertically into the substrate and horizontally to lower-temperature surface regions. The process may be referred to as thermal flux annealing.
The three types of annealing can be distinguished in thermodynamical terms. RTP and thermal annealing are isothermal processes in which every region of the wafer is at essentially the same temperature at a given time. Pulsed laser annealing is adiabatic. The radiation pulse has ended before any heat can significantly diffuse. Thermal flux annealing is faster than the isothermal RTP process but slower than the adiabatic pulsed process. Heat has a thermal diffusion length of between 5 and 100 μm in conventional electronic materials, a length which allows some thermal homogenization on the scale of integrated circuit patterning.
Markle et al. (hereafter Markle) discloses a reflective-optics version of such a linear scanning thermal processing system in U.S. Pat. No. 6,531,681. Jennings et al. (hereafter Jennings) discloses refractive-optics versions in U.S. Published Application 2003/0196996, although there are other substantial differences between Markle and Jennings In some embodiments, the Jennings thermal apparatus can achieve ramp rates of 106° C./s with beam line widths of less than 100 μm.
However, both Markle and Jennings prefer the use of laser diode bars lined up along the long direction of the beam to produce laser radiation These laser diode bars are typically composed of GaAs or similar semiconductor materials and are composed of a number of diode lasers formed in a same layer of an opto-electronic chip. The GaAs laser bars preferred by Markle emit near-infrared radiation at a wavelength of about 808 nm, which couples well into silicon. As illustrated in the energy band diagram of FIG. 1, semiconducting silicon like most semiconductors has a valence band 10 of electron states with energies lower than Ev and a conduction band 12 of electron states at energies above Ec. In direct bandgap semiconductors, a bandgap 14 of energy Eg separates the valence and conduction bands 10, 12. In undoped silicon, no electron states exist in the bandgap 14. For silicon, Eg=1.12 eV, which corresponds to an optical wavelength λg of 1110 nm according to the well known photon equation
      E    =                  h        ⁢                                  ⁢        c            λ        ,where h is Planck's constant and c is the speed of light. At a temperature of absolute zero in an indirect-bandgap semiconductor such as silicon, the valence band 10 is completely filled and the valence band 12 is completely empty.
Light having a photon energy of Ep passing through such a semiconductor will interact with the electrons only if its photon energy is greater than or equal to the bandgap,Ep≧Eg so that the photon can excite an electron in the valence band 10 to the conduction band 12, where it is a free carrier. Once the electron is in the conduction band, it quickly thermalizes and heats the semiconductor body.
The situation changes when the silicon is heated to a high temperature at which thermal energy excites electrons from the valence band 10 to the conduction band 12 leaving holes (empty electron states) in the valence band 10 and electrons in the conduction band 12, both of which are free carriers. Lower energy photons can excite valence electrons into the holes within the valence band 10 or can excite thermally excited conduction electrons into the generally empty states within the conduction band 12. However, this effect is generally small below about 800° C. Another effect arises when the semiconductor is doped, either with n-type dopants to produce electron states 16 within the bandgap but close to the conduction band 12 or with p-type dopants to produce hole states 18 close to the valence band 10. These dopant states are important for the operation of semiconductors because at moderate temperatures they are sufficient to excite the electron states 16 into the conduction band 12 or hole states into the valence band 10 (which can be visualized alternatively as exciting a valence electron into the hole state 18). Lower-energy photons can interact with such excited dopant states. For example, intra-band transitions resulting in absorption of the incident radiation may occur between two free-carrier states within the valence band 10 or within the conduction band 12. However, the absorption provided by this effect is relatively small below doping levels of about 1018 cm−3, far above the average doping level in semiconductor devices. In any case, the laser absorption should not critically depend upon temperature and the doping level of the irradiated area, as is the situation with intra band absorption between free carriers. It is preferred to rely on inter-band transitions for laser heating rather than upon intra-band absorption involving free carriers for which temperature and doping levels have profound effects.
Hence, laser radiation for rapidly heating silicon should have a wavelength substantially less than 1110 nm, which is easily provided by GaAs diode laser. Diode lasers, however, suffer several drawbacks. Laser bars create a problem in focusing their output into a beam uniform along its length. The radiation from a laser bar is output separately from a number of diode lasers spaced along the length of the bar with gaps between them. That is, the linear uniformity at the laser source is not good and needs to be improved by an homogenizer. The technology for homogenizers is available, but applying them to high intensity beams presents engineering and operational problems. A further problem is that laser bar radiation at 808 nm has an absorption depth of about 800 nm in silicon, which may be greater than the depth of the silicon layer requiring annealing, such as shallow source and drain implants needing implant curing and dopant activation.
In U.S. Pat. No. 6,747,245, Talwar et al. (hereafter Talwar) suggests using radiation from a carbon dioxide (CO2) laser to produce line beams for laser thermal processing. Although CO2 lasers have a lower efficiency (10 to 15%) than diode lasers (40 to 50%), they can more easily produce a well collimated (non-divergent) and generally circular beam. However, we believe that CO2 radiation having a wavelength of about 10.6 μm is ineffective as the sole source of laser radiation since the 10.6 μm wavelength is much greater than the silicon bandgap wavelength of 1.11 μm. As a result, undoped or low-temperature silicon is virtually transparent to CO2 radiation and the CO2 radiation is not effectively absorbed in a silicon wafer, more or less its shallow surface region desired for advanced microprocessing. Although not disclosed by Markle, absorption of the CO2 radiation may be enhanced by heating the silicon to either to a very high temperature or by relying upon heavy doping or a combination thereof. However, the heating apparatus complicates the laser thermal processing apparatus, and the doping levels cannot be freely controlled in semiconductor fabrication and vary across the partially developed integrated circuit.
Boyd et al. (hereafter Boyd) discloses a dual-wavelength thermal processing technique in “Absorption of infrared radiation in silicon,” Journal of Applied Physics, vol. 55, no. 8, 15 Apr. 1984, pp. 3061-3063. Boyd emphasizes that the quantum energy of 10.6 μm radiation is two orders of magnitude smaller than the silicon bandgap. As a result, silicon is essentially transparent to CO2 radiation. Even for heavily doped silicon, the absorption coefficient is less than 100 cm−1, a value too small for surface laser thermal processing. Instead, Boyd proposes either pre-heating the silicon or more preferably irradiating the silicon with 500 nm radiation from a CW argon laser, having an energy greater than the bandgap, to increase the free carrier density in silicon and promote absorption of CO2 radiation. Boyd does not address the spatial extent of his beams and admits to poor spatial definition, issues that are crucial for advanced laser thermal processing.